Ruby and Sapphire: Gems of the Mineral Corundum Home » Gemstones » Ruby and Sapphire Ruby and Sapphire Ruby and sapphire have the same chemical composition and crystal structure - they are both varieties of the mineral corundum Rubies: The most desired variety of corundum is the ruby. The red color is produced by trace amounts of chromium in the mineral. These two beautiful rubies were mined in Madagascar. The one on the left is a 7 x 5 millimeter octagon that weighs about 1.32 carats. The one on the right is an 8 x 6 millimeter oval that weighs about 1.34 carats. Although Asia has been the traditional source of gem corundum for over one thousand years, Africa is poised to become a new primary source. Ruby, Sapphire, and Fancy Sapphire Most people don't realize that ruby and sapphire are both gems of the mineral corundum . Both of these gems have the same chemical composition and the same mineral structure. Trace amounts of impurities determine if a gem corundum will be a brilliant red ruby or a beautiful blue sapphire. It is surprising that "impurities" can produce such wonderful results!

Red and blue are just two of the many colors found in gem corundums. Trace amounts of other elements can produce brilliant yellow, orange, green, and purple gems. Red corundums are known as "rubies," blue corundums are known as "sapphires," and corundums of any other color are known as " fancy sapphires ." Corundum produces gems in a spectrum of colors. Related: Corundum: Mineral Summary What Makes a Ruby? Some gem-quality corundum contains trace amounts of chromium. A very small amount of chromium gives corundum a pink color. Larger amounts of chromium increase the color saturation of the stone and produce a gem with a deeper red color. To be considered a "ruby," a corundum should have a color between orangey red and a slightly purplish red. The most desirable color is a pure vibrant red. Very few specimens of corundum have a natural color within the range required for a ruby. Very few also have the clarity required to produce a nice faceted stone. Long ago, people who prepared gem materials for cutting discovered that heating rubies under controlled conditions can intensify their color.

Heating can also cause inclusions to become less visible and improve the clarity of a gem. Most rubies in the market today have been heated to improve their color and clarity. Many of them have had other treatments to improve their appearance. These treatments are normal and expected in the gem trade, but a seller should disclose them to a buyer in advance of a sale. Montana sapphire: The most widely known sapphire locality in North America is Yogo Gulch, Montana, famous for producing deep blue sapphires of excellent quality. Creative Commons photo of a gem from Barnes Jewelry, Helena, Montana, by Montanabw. Corundum as ruby, sapphire, and fancy sapphire: Gem-quality corundum is a highly prized and valuable material. When it is bright red in color, it is called "ruby." When it is deep blue, it is called "sapphire." Gem-quality corundum of any other color is called "fancy sapphire." All of the stones in this photo were mined in Africa. What Makes a Sapphire? Trace amounts of iron and titanium can develop a blue color in corundum.

Blue corundums are known as "sapphires." The name "sapphire" is used for corundums that range from light blue to dark blue in color. The blue can range from a violetish blue to a greenish blue. Stones in the middle of this range, with a rich blue color, are the most desirable. Gem-quality corundum occurs in a wide range of other colors, including pink, purple, orange, yellow, and green. These stones are known as "fancy sapphires." It is surprising that a single mineral can produce gemstones of so many different colors. When the color of a sapphire is any color other than blue, the color should be used as a preceding adjective to describe the stone. For example, pink sapphire, yellow sapphire, or green sapphire. Used alone, the word "sapphire" refers only to blue corundum. The color and clarity of blue and fancy sapphires can be altered by heat, radiation, and other treatments. Sometimes pale, milky, cloudy, or translucent stones can be heated to yield stones that are brighter in color and more transparent.

If such treatments have been used, sellers should disclose the types of treatments to potential buyers when the stone is presented for sale. Ruby on marble: A ruby crystal on white marble from Jegdalek, Saroby, Afghanistan. This crystal is about 1.6 centimeters in length. Specimen and photo by Arkenstone / . Sapphire crystal: A blue, doubly terminated, translucent sapphire crystal from Sri Lanka. Most crystal specimens like this have been transported by streams and show a greater amount of wear. This specimen has not been treated. Heat treatment would likely deepen the color, make it more uniform, and improve the clarity. This crystal is about five centimeters in length. Specimen and photo by Arkenstone / . Popularity of Ruby and Sapphire Ruby and sapphire are extremely popular gemstones. Virtually every jewelry store that features colored gemstones in jewelry will have a generous portion of their display dedicated to ruby and sapphire items. Ruby is the most popular red gemstone, and sapphire is the most popular blue gemstone.

The pie chart below shows the share of colored stone imports on a dollar value basis that went to the categories of sapphire, ruby, emerald, and all other gemstone varieties during the 2011 calendar year. It shows that sapphire and ruby were the second and third most imported colored stones during that year. A total of $282 million worth of sapphire was imported, and a total of $45 million worth of ruby was imported. Colored stone imports: This chart illustrates the popularity of sapphire and ruby in the United States. The pie represents all colored stones imported into the United States during 2011 on the basis of dollar value. As single gem varieties, sapphire and ruby hold major positions in the import market, accounting for over 35% of all colored stones imported and a total value of approximately $400 million. Data is from the USGS Minerals Yearbook, April 2013. [5] Although the pie chart above does not include domestic colored stone production, it can be considered as nearly complete.

The United States Geological Survey estimates that the value of the total domestic production of colored stones of all kinds in the United States during calendar year 2011 was only $11 million. Related: Emerald: The Most Popular Green Gem Synthetic star ruby: Laboratories have been able to mass-produce synthetic star corundum since the Lindy division of Union Carbide flooded the gem market with them in the 1950s and 1960s. This synthetic red corundum has a visible six-ray star and a faceted back to enhance the base color of the stone. Star sapphire: Some specimens of sapphire and ruby contain a very fine "silk" of fibrous inclusions that parallel the crystallographic axes of the mineral. When these stones are cut into a cabochon with the c-axis of the stone penetrating the base of the stone at right angles, a six-rayed star can be seen floating on the surface of the cabochon. These are known as "star sapphires" or "star rubies," according to their color. Public domain photography by Mitchell Gore.

Mining Rubies and Sapphires Most gem-grade corundum forms in metamorphic rocks such as schist or gneiss and igneous rocks such as basalt or syenite. However, gem corundums are rarely mined from the rocks in which they form. Mining small gems from hard rock is possible, but it is very expensive, and many of the gems are broken during the mining process. Fortunately, corundum is very hard and resistant to weathering. In many areas, natural weathering and erosion have liberated the stones and carried them into streams over long periods of geologic time. Today, the gems are mined from these stream sediments. Their high specific gravity relative to other sediment particles often causes currents to concentrate them in small placer deposits. Most rubies and sapphires are produced by washing the gravels of these stream deposits. This work is often done by hand because the deposits are small and irregular in shape and character. These deposits are often located in countries when wages are very low and artisanal mining is prevalent.

Noteworthy locations where gem-quality corundums have been produced include Myanmar, Thailand, Cambodia, Vietnam, India, Pakistan, Afghanistan, Sri Lanka, China, Australia, Madagascar, Kenya, Tanzania, Nigeria, and Malawi. Blue star sapphire: A deep blue star sapphire 8 mm x 6 mm cabochon from Thailand. Inclusions within the stone align with the crystallographic axis to produce the star - which is only clearly visible and centered when the back of the stone is cut at 90 degrees to the c-axis of the crystal. This stone has been heat treated to darken the stone and enhance the visibility of the star. Ruby and Sapphire Information [1] Gems: Their Sources, Descriptions and Identification: Michael O’Donoghue; Elsevier; sixth edition; 873 pages; 2006. [2] Gemstones of the World : Walter Schumann; Sterling Publishing; fifth edition; 320 pages; 2013. [3] Rubies and Sapphires : Fred Ward; Fred Ward Gem Books; 65 pages; 2010. [4] Rock and Gem : Ronald L.

Bonewitz; The Smithsonian Institution, Dorling Kindersley Publishing; 360 pages; 2008. [5] 2011 Minerals Yearbook: Gemstones : Donald W. Olson; The United States Geological Survey; 22 pages; 2013. Synthetic Corundum as an Essential Product Rubies and sapphires have been highly sought after in many parts of the world for over one thousand years. Deposits that produce high-quality stones of good color have attracted enormous amounts of attention and have been heavily exploited. As a result, buyers who need large quantities of quality stones are having a harder time finding them in the volumes needed for today's jewelry marketplace. Let's imagine a jewelry manufacturer who wants to create enough matching ruby pendant, ring, and earring sets to supply a large jewelry chain with thousands of stores and a busy internet site. This manufacturer will need at least four nice rubies for each matching set, multiplied by enough sets to supply thousands of stores and a busy internet site. This manufacturer will need millions of rubies, all color-matched into sets and all cut into calibrated shapes and sizes.

On the production side, the labor needed to discover, mine, grade, cut, and polish these stones will be enormous. An enormous effort will also be needed on the manufacturing side just to find enough sellers to provide them, confirm their quality, negotiate prices, make large numbers of purchases, and deliver the stones to the manufacturing facility. The problem of sourcing gemstones to meet the needs of a large manufacturing company is so much simpler if the stones themselves are manufactured. This is why laboratories capable of reliably producing synthetic rubies and sapphires of consistent size, color, grade, and appearance have found an important place in the gemstone market. If you go shopping in the United States and look at the ruby and sapphire jewelry offered in many famous-name mall jewelry stores and department stores in the $50 to $500 price range, you will find that many of the jewelry items are made using "created" or "synthetic" rubies and sapphires. The synthetic corundum in this jewelry is perfect in color, has wonderful clarity and is extremely attractive. Many shoppers see the lower price and better appearance of the synthetic materials when compared to natural stones of similar size and opt to purchase the synthetic.

It is a logical choice based upon what appeals to the person and what they are willing to pay. They get great appearance at a lower price. Natural gemstones are a finite resource that will become more difficult to obtain and more expensive over time. As a result, buyers will probably see more synthetic stones offered in most jewelry stores and should expect to see the price difference between synthetic stones and natural stones of similar size, color, and quality become greater in the future. Many consumers prefer natural stones, and many also make the decision to purchase synthetic stones. That follows the often-given advice of "buy what you like and can afford." Corundum: The mineral Corundum, Sapphire, Ruby info & pictures Advertising Information The Mineral corundum Corundum is best known for its gem varieties, Ruby and Sapphire. Ruby and Sapphire are scientifically the same mineral, but just differ in color. Ruby is the red variety, and Sapphire is the variety that encompasses all other colors, although the most popular and valued color of Sapphire is blue. Sapphire is also only used to describe the gem variety; otherwise it is simply called Corundum. Corundum is a very hard, tough, and stable mineral.

For all practical purposes, it is the hardest mineral after Diamond , making it the second hardest mineral. It is also unaffected by acid s and most environments. Translucent brown Corundum and Emery are the most common forms of Corundum. These are fairly common forms, and due to their great hardness and prevalence are the most favorable abrasive s. The industrial term "emery" describing Corundum abrasives is derived from the variety Emery which is mined specifically for its use as an abrasive. Erosion may cause Emery to crumble and form sand, which are sometimes called "black sands." Corundum is easily synthesized , and many Corundum abrasives are synthetic. Synthetic gems are also easily created by adding traces of certain color producing elements to the Corundum solution, and letting the solution solidify into a boule , or synthetic, unprocessed "mineral" with a particular shape. This process is called the Verneuil process . Natural Ruby and Sapphire gemstone s may have their color artificially enhanced or deepened through heat treatment when used as gems. Some dark blue stones from certain localities may also be made a bright blue desirable color. USES Ruby and Sapphire are the most famous gemstones after Diamond .

They have retained their high value throughout the centuries and are continuously and vigorously mined throughout the world. Red rubies and blue Sapphires are the most popular Corundum gemstones, though other Sapphire colors such as orange, yellow, and green are becoming increasingly popular in gemstone use. Corundum has some electrical uses, and non gem material, such as Emery , is used as an abrasive because of its high hardness . The gemstone applications of the Corundum varieties are discussed in greater detail in their own dedicated gemstones pages. Make sure to also see the gemstone section on Ruby and Sapphire . NOTEWORTHY LOCALITIES The area of Mogok, Burma (Myanmar) is the source of some of the best gem -quality Ruby . Another significant Burmese deposit is Mong Hsu, which is currently a major Ruby producer, though its color isn't as fine as the Mogok material. Sri Lanka, especially the area of Ratnapura, Bibile, and Rakwana, is an excellent source of gem quality Corundum of all colors. The finest and largest crystallized specimens and aggregate s of gem-quality Sapphire are from Sri Lanka.

Ruby with a brownish tint comes from Thailand, in the Chantaburi District. Fine gem Ruby and Sapphire comes from Luc Yen, Yenbai Province, Vietnam; the Hunza Valley, Gilgit, Pakistan; and Jegdalek, Sorobi District, Afghanistan. Gem quality Sapphire is well-known at Pailin, Cambodia. Translucent to opaque Ruby, sometimes with asterism , is very abundant at Mysore (Mysuru) and Subramanium, in Karnataka, India. Africa has recently become a significant producer of Corundum, especially in Madagascar, where it is found in the Zazafotsy Quarry, Ambahatraso; and in Andranondambo, Amboasary District. A famous African locality in which opaque rubies are found in green Zoisite is is in Longido, Kilimanjaro Region, Tanzania. Another important Tanzanian Ruby mine is Winza, in Arusha. In the U.S., the Yogo Gulch in Judith Basin Co. has produced choice, deep blue Sapphire crystals. Not far from the Yogo Gulch, near Helena, waterworn Sapphires stones are found in the Missouri River throughout its length in Lewis and Clark County.

Montana is also the claim to a few other localities: the Gallatin Gateway and Salesville, Gallatin Co.; Rock Creek, Granite Co.; and Cottonwood Creek, Deer Lodge Co. In California, well-formed, elongated Corundum crystals were found in Cascade Canyon in the San Gabriel Mountains, San Bernardino Co. A well known Ruby locality is the Cowee Creek District, Macon Co., North Carolina, especially the area of Franklin. Large deposits of Emery were once worked near Peekskill, Westchester Co., New York; and Chester, Hampden Co., Massachusetts. An important Canadian occurrence of Corundum is the Burgess Mine, Bancroft, Hastings Co., Ontario. Gemstone Characteristics Colour We believe that the most important single gem characteristic is colour. Gems such as ruby , amethyst , sapphire and emerald have attractive colour as their main attraction, and are generally expected to be transparent, however it is not necessary for gemstone to be transparent. Coral, lapis lazuli, cornelian and opal can all be considered beautiful, but are usually opaque. Colourless stones, naturally, are bound to be less attractive or beautiful, or are they?

Although sapphire, spinel and quartz and other gemstones can occur in colourless varieties, they are usually quite uninteresting because they do not have the advantage of possessing the beauty of colour. Diamond likewise is often colourless, but perversely, it is considered by many to be the most desirable gemstone. The main reason why diamond is suitable as a gemstone is its high refraction and dispersion - now we are getting technical! Let's consider colours for a moment rather than coloured gemstones. Some colours are known to be high-preference colours, others will have high attention value. Yellow appears to have the highest attention value. Perception tests have shown that people will see yellow objects more quickly than other colours, brown and green are lower attention colours. Other colours are high preference colours, blue is probably the highest preference colour, with red, purple and green following. Naturally whatever people regard as their favourite colour is likely to be high on their desirability list when it comes to gemstones. This last fact makes it even more odd that many people's top choice of gemstone is diamond!

Sparkle Actually it's necessary to consider why some stones sparkle more than others:- Reflection Dispersion Facetting Let's take these four concepts in turn, although as you will see there is some interaction between them. Reflection All stones will reflect light from their surface. The amount of light reflected will depend on the surface reflectivity of the stone. Although the reflectivity is related to refractivity, factors such as the hardness of the stone affect its reflectivity, partly though the ability of hard stones to accept a high polish. The reflectivity of a stone at its surface is often referred to as its lustre. Transparent stones will also reflect light from their internal surfaces. How strongly they do so depend on their refractive index. The proportion of light reflected off an internal surface increases suddenly at a particular angle for any pair of materials. This angle is known as the angle of total internal reflection. This angle is very important when determining the optimum cutting angles of different gemstones. Ideally all the light entering the stone from any angle would escape via the top (front) of the stone. The higher the refractive index of the stone, the more light will be reflected internally from the back facets. Refraction All materials refract light (alter its angle).

The amount by which light is refracted is an important feature of every gemstone. The measure of a material's refactivity is known as its refractive index or R.I. Without going too deeply into physics, light travels at different speeds through different media. When light passes from one medium to another, its path is altered (bent) and it is said by by diffracted. Dispersion Dispersion is the difference between the amount of refraction of different colours of light. White light is actually composed of light of all different colours. A highly dispersive material will split light strongly into its component colours to give a "prism" effect showing a "rainbow" or spectrum. Dispersion is often know as "fire". Diamond owes most of its beauty to its high dispersion, whereas white sapphire, with low dispersion tends to look "dead" or glassy. Facetting The number of facets on a gemstone, and their careful placement will tend to give a stone more sparkle, and enhance its beauty and desirability. Size In general, the larger a gemstone, the more obvious its beauty, and therefore the greater its desirability.

A microscopically small gemstone may be of interest to a gemmologist perhaps because of its rarity, but would be of little practical use in jewellery. Obviously it is also possible for a gemstone to be too large to be of practical use in jewellery, in which case it could be recut into a number of smaller stones. Shape To some, a heart shaped stone may be more desirable than a different shape, but this is usually a matter of personal preference. Some stones naturally occur in certain typical shapes, or lend themselves to be cut in certain shapes. Emeralds for example lend themselves to being cut into oblong or octagonal shapes, so that an oblong shape with its corners cut to form an octagon is known as an emerald cut. Durability Regardless of how beautiful a stone may appear, for it to be suitable as a gemstone, it will need to be reasonably durable. Gemstones do vary considerably in their durability, some, such as diamond, are almost indestructible, others are considerably more delicate. Indeed, one paradox of gemstones is that something so small and delicate-looking can be so strong. It is important to realise that durability is not the same as hardness.

For example, try bouncing a glass bottle and a rubber ball on a concrete floor. Glass is harder than rubber, yet the glass bottle smashes, and the rubber ball bounces. The rubber ball is more durable. Hardness There are numerous different ways of considering hardness, but the main one for gems is their resistance to abrasion. Moh's hardness scale lists 10 well known minerals on a scale from 1 to 10. Each numbered mineral will scratch the mineral numbered below it. Diamond is shown as 10 on Moh's scale, and is the hardest naturally occuring substance. Even though diamond is very tough, it can be shattered by a hard blow, especially at an unlucky angle. Corundum, of which ruby and sapphire are composed, is the next on Moh's scale at 9. Rubies and sapphires are both hard and tough. Topaz is listed at 8, so is very hard, but it is relatively easy to break topaz, especially along its cleavage planes. It is said to possess perfect cleavage, and expression which always stirs my imagination!

Quartz, which includes amethyst and citrine, is listed as 7, and is the softest translucent material which is recommended for general jewellery usage as a gemstone. This is because most dust contains tiny quartz crystals, and because its hardness is 7, will scratch softer materials. Emerald has a hardness of about 7.5, chrysoberyl 8.5, garnet 7.25 to 7.5, zircon 7.5, all these are of sufficient hardness to be useful as facetted gems. There are a few gemstones which are transparent, and therefore usually facetted, and because of their other desirable properties, are frequently used in jewellery, and can be worn with care. These include sphene 5 to 5.5, zoisite including tanzanite 6, demantoid garnet and some zircons at 6.5, and peridot at 6.5 to 7. Gems which are opaque do not show surface scratches as badly as facetted transparent stones, and therefore do not need to be as hard. Most opaque stones are polished into cabochons, with smooth rounded surfaces, as opposed to flat facets.

Such gemstones include amber 2 to 2.5, pearl 2.5 to 3.5, coral 3.5, opal 5.5 to 6, and turquoise at 6. Transparent gems can also be cut as cabochons, although coloured gemstones make much better cabochons than do colourless stones. Rarity Many authors and jewellers state that rarity is necessary or desirable for a gemstone. We believe that a beautiful and attractive gemstone is just as beautiful and just as attractive whether it is common or rare. Supply and demand usually dictate that rare objects command high prices. In our opinion, amethysts are often underrated despite their beautiful colour, because they are relatively inexpensive. Only the status value of gems is increased by their rarity, not their aesthetic value, although there are mineral collectors who appreciate the value of possessing rare and unusual specimens whether beautiful or not. Not all common gemstones are cheap, and some rare gemstones are probably undervalued. Diamonds, for example must be about a million times more common than alexandrite. Natural, Synthetic or Simulant? Again, many jewellers and jewellery consumers, insist that being natural is a necessary attribute for a gemstone.

We do not completely agree with this. Certainly, we understand that ownership of a rare, expensive natural gemstone gives it owner a degree of status and personal satisfaction, but we don not believe that this makes a beautiful and attractive synthetic or simulated gem any less beautiful. Our view may be a minority view amongst members of the jewellery trade, but we believe that the prime purpose of jewellery is, or should be, adornment. We believe that the elitist stance of many jewellery professional is because they perceive synthetics as a threat to their profits and livelihoods. It is difficult to sell an expensive diamond to a customer who realises that synthetic gem quality diamonds can be manufactured, so it is only natural for many jewellers to resist the threat posed by progress. We look forward to the day when we can offer a gem quality synthetically produced 1 carat diamond for �100! We believe there will still be a demand for the natural product also. There is a difference between natural, synthetic, and simulated. Not all synthetics are simulants, and not all simulants are synthetic.

A gem simulant is a a stone which appears similar or identical to another, different stone, so that cubic zirconia and natural colourless sapphire can both be used to imitate or simulate diamond. A synthetically produced diamond is a real diamond, not a simulant for diamond. Synthetic sapphire can be used to simulate alexandrite (not very convincingly), but can never simulate sapphire because it is sapphire. Gemstone simulants have been made for about 7000 years. The ancient Egyptians used to make a turquoise coloured ceramic called faience in imitation of turquoise. Synthetic gemstones are relatively recent. Synthetic ruby has been produced since before 1905, spinel from 1908, sapphire from 1911, and diamond from about 1950. Cultured pearls are a simulant produced by a natural process induced by human interference. A cultured pearl is one which has been produced by an oyster, or other bi-valve, by inserting a bead, usually mother-of-pearl, into the oyster. The oyster then proceeds to coat the bead with nacre, from which natural pearls are also formed.

Fashion Fashion can play a part in determining whether a material is regarded as a gemstone. Moonrock is quite rare and although quite unattractive, yet it would be considered a fashionable and desirable gemstone, especially if some famous person were to be photographed wearing it, and little else, to the premier of a high budget Hollywood film at the Cannes Film Festival. Ivory is currently out of fashion as a gemstone. Yellow and brown tinted diamonds are considered to be inferior to colourless ones, and so, in most ignorant folk's opinion would intensely coloured natural fancy canary coloured ones. We believe that coloured diamonds may become the next jewellery fashion trend. We have always liked liked them, but they are not well appreciated in our corner of England. We also belive that De Beers will never dare to promote them because it might lead to people realising the beauty of coloured stones, whereas most diamonds are almost colourless. Special Properties By special properties, we include the possible health or social effects which gemstones can possess, so we include the following categories of superstition, religion, social custom, and health.

Health It has long been believed that wearing certain minerals and gemstones can create beneficial health effects, amethysts for example were regarded as protecting the wearer from becoming drunk. Currently, we notice that there are many claims that unfacetted crystals can produce vibrations which protect the wearer from radiation or other hazards. We remain somewhat cynical but tolerant, and open-minded, about these claims. Superstition Superstition plays a part, we hear many people who belive that emeralds or opals are unlucky, or that "pearls are for tears". We believe that most stories such as this are ill-founded. Of course if an individual believes that a certain stone is unlucky for them, then there is little point us trying to convince them otherwise. Also if an individual believes a particular stone brings them luck, then we would not seek to upset their viewpoint. It this increases their self-esteem or confidence, then it may actually improve their quality of life. We strongly believe this to be a good thing, if gemstones and jewellery can create such a beneficial effect, perhaps it should be available on the National Health.

(Of course, we don't know what would happen to waiting lists!) Religion Bishops wear large stone set rings as a symbol of their office, usually red or purple stones. Similarly red stones set in a cross or crucifix may have a special religious significance. Different religions will also have similar practices. Social Customs In some societies and cultures, the wearing of gemstones, perhaps shells or beads, as jewellery may convey messages about the social status of the wearer. Summary The following chart could be used as a visual summary of the relationship between different desirability factors of gemstones. Beauty Sapphire Quality Factors Sapphire Quality Factors This sapphire from Kashmir has an intense velvety blue color. - Courtesy George C. Hartmann, Jr. Sapphire is one of the Big 3 of jewelry colored gemstones—the other two are ruby and emerald. A durable stone that’s designated as a birthstone for September, it captures jewelry buyers with its practicality and aura of romance. Color Sapphires come in a wide range of colors, and each color has its own quality variations. In general, the more intense the color and the fewer the distracting zones of unattractive color, the more valuable the stone.

Color has the most important influence on blue sapphire’s value. The most highly valued blue sapphires are velvety blue to violetish blue, in medium to medium dark tones. Preferred sapphires also have strong to vivid color saturation. The saturation should be as strong as possible without darkening the color and compromising brightness. Sapphires with these qualities command the highest prices per carat. Madagascar is a relative newcomer as a major sapphire source, but its stones can have an intense blue color that rivals the finest from more traditional sources. - © GIA & Tino Hammid At the other end of the price scale are commercial-grade sapphires with greenish blue color or strong greenish blue pleochroism. Pleochroism is different colors seen in different crystal directions. Less valuable blue sapphires might be grayish, too light, or too dark. The major fancy sapphire color categories are padparadscha, pink and purple, orange and yellow, green, and colorless and black. Each category has its own color range, causes of color, and market. The fancy sapphires that people in the trade call padparadscha are very beautiful.

They typically have a high per-carat value, too—much higher than other types of fancy sapphires. Their color can be hard to describe. Some people say padparadscha sapphire colors should be called salmon or sunset. Others compare the color to the flesh of a ripe guava. In spite of these differing color descriptions, people in the industry usually agree that padparadscha sapphire colors are intensely saturated and range from light to medium pinkish orange to orange-pink. Pink sapphires range from red to purple with weak to vivid color saturation and lighter tone. Purple sapphires are similar in color but darker and always have purple as the dominant color. They range from medium to dark reddish purple to violetish purple with weak to vivid color saturation. Some pink sapphires have intensely saturated color. - John Dyer, gem courtesy of John Dyer & Co. Corundum appears in an array of yellow and orange hues that includes bright lemon, soft peach, and vivid tangerine. In specific color terms, yellow sapphires range from light to dark greenish yellow to orangy yellow with weak to intense color saturation. The finest yellow sapphire is yellow to orangy yellow with vivid saturation.

Orange sapphires range from yellowish orange to reddish orange. The finest orange sapphires are strong, pure orange to red-orange with medium tone and vivid saturation. These sapphires range from yellow to a deep yellow-orange color called golden to highly saturated reddish orange. - © GIA & Tino Hammid, courtesy Varujan Arslanyan Green sapphires range from light to dark bluish green through yellowish green, and are usually low in saturation. Green sapphire is readily available, but its color isn’t very marketable. Its color is sometimes described as khaki or olive. That’s because the stones tend to have low saturation or unattractive color zoning. People in the trade refer to corundum in its purest form as either colorless sapphire or white sapphire. The closer corundum comes to having no color, the more valuable it is as a colorless sapphire. Traces of extremely light gray, yellow, brown, and blue are common, and reduce the value. Colorless sapphires have been popular as small accent stones in jewelry. Color-change sapphires are corundum’s chameleons—stones that change color under different lighting.

Under daylight equivalent (fluorescent) light, the typical color-change sapphire’s basic color ranges from blue to violet. Under incandescent light, it ranges from violetish purple to strongly reddish purple. Some rare color-change sapphires change from green in daylight to reddish brown in incandescent light. When gem experts judge color-change sapphires, they describe the color change as weak, moderate, or strong. The strength of the stone’s color change is the most important quality factor affecting its value, followed in importance by the actual colors of the stone. As with all colored stones, the color of star corundum has a great effect on its value. The best star corundum has a crisp, distinct star against strongly saturated color. If the color is too light, it doesn’t provide enough contrast for the star’s rays, and the star will be less visible. Star corundum can be red, blue, black, gray, purple, or yellow—practically every color under the sun. The term “star sapphire” encompasses all colors of star corundum except red, which is called star ruby. Naturally, some colors of star corundum are valued more highly than others.

In general, the most prized colors are the same as the colors most valued in non-phenomenal corundum: red and blue. Trade terms based on sources can represent certain colors and qualities that are associated with a stone’s source. The qualities might be typical of that source or they might represent the finest stones from that source. But a single source never consistently yields gems that are all the same color and quality. In fact, the descriptive term might represent only a small percentage of stones from that source. The appearance of stones from a particular source often varies over time, and the original quality associated with that source might no longer match the material produced. New sources can produce material very similar to that from classical sources or with a slightly different, but just as beautiful, appearance. Clarity Blue sapphires typically have some inclusions, but they generally have better clarity than rubies. Blue sapphires with extremely high clarity are rare, and very valuable. Several types of inclusions are found in sapphires. Among these are long thin mineral inclusions called needles. Fine needles are called silk when they occur as the mineral rutile in intersecting groups.

Other clarity characteristics in sapphire are included mineral crystals, partially healed breaks that look like fingerprints, color zoning, and color banding. These long, thin, intersecting inclusions of the mineral rutile are called silk. This arrangement can produce the star effect in corundum. Generally, inclusions make a stone less valuable. Price can drop substantially if the inclusions threaten the stone’s durability. Even so, inclusions can actually increase the value of some sapphires. Many of the most valuable Kashmir sapphires contain tiny inclusions that give them a velvety appearance. They scatter light, causing the coveted visual effect without negatively affecting the gem’s transparency. Star rubies and star sapphires belong to the phenomenal corundum category. The star effect is called asterism. It’s caused by reflections from tiny, needle-like inclusions that are oriented in several specific directions. Stars are usually made up of 2, 3, or 6 intersecting bands, resulting in 4, 6, or 12 rays. Tiny inclusions create magnificent reflections of light known as the star effect.

- © GIA & Tino Hammid, courtesy Benjamin Zucker The most common stars have 6 rays, and 12-rayed stars are quite rare. Two different sets of inclusions—one of rutile and one of hematite—oriented in slightly different directions can cause a 12-rayed star. Hematite inclusions cause asterism in black star sapphires. The sapphire’s color is actually yellow, green, or blue, but the inclusions make it appear dark brown or black. The finest star is distinct, centered on the top of the stone, and visible from a reasonable distance, about arm’s length. The star’s quality should be the same when viewed from all directions. The rays should be uniform in strength, reach from one side to the other, and intersect at the top of the stone. They should be straight, not fuzzy, wavy, or broken, and they should contrast strongly against the background color. The star should also have elegant “movement.” This means that, as you rock the stone, the star should move smoothly across the surface with no dead spots. The best and most expensive star corundum is semi-transparent, with just enough silk to create a well-defined star.

Too much silk can harm transparency and also lead to poor color, lowering the value of the stone considerably. Cut Oval shapes with triangular and kite-shaped facets on the gem's crown (top portion) and parallel rectangular facets on the gem's pavilion (bottom portion) are very common for corundum of all colors. The shape of a rough sapphire crystal influences the finished stone’s shape and size. Rough sapphire’s most common crystal form is a barrel- or spindle-shaped hexagonal pyramid. For this reason, finished sapphires are often deep. To achieve the best overall color, maintain the best proportions, and retain the most weight possible, cutters focus on factors like color zoning, pleochroism, and the lightness or darkness of a stone. These sapphires are miniature works of art that have been expertly cut into custom designs. - Courtesy Mark Gronlund, Umatilla, Florida Color zoning—areas of different colors in a stone—is a common sapphire characteristic. Blue sapphire often has angular zones of blue and lighter blue. To accommodate color zoning in some sapphires, cutters orient the concentrated color in a location that offers the best visible color in the cut stone.

In Sri Lankan sapphires, the color is often concentrated close to the surface of the crystal. If a cutter can orient the culet within the concentrated area of color, the stone will appear entirely blue in the face-up position. Pleochroism is different colors in different crystal directions. Blue sapphires often have greenish blue and violetish blue pleochroism. It’s most desirable to orient the cut so the stone shows the violetish blue color when it is set in jewelry. Besides coming in all colors, corundum also comes in all cutting styles. - Jeff Scovil, courtesy Ballerina Gems Star corundum must be cut as a cabochon to display asterism. A finished stone’s attractiveness depends on the star’s orientation and the cabochon’s symmetry, proportions, and finish. The cabochon must have an appealing appearance, with the star properly centered when the gem rests on its base. The stone’s outline should be symmetrical. For most stones, the dome should be fairly high—about two-thirds of the stone’s width—to focus the star sharply. If it’s too high, the phenomenon loses its graceful motion when the stone is tilted.

Excessive height also makes the stone difficult to mount. If the dome is cut too shallow, the star will be visible only from directly above. Black star sapphires, however, are prone to parallel breaks, so they’re usually cut very flat to reduce the risk of damage. A stone should not have excess weight below the girdle that doesn’t contribute to the optical effect or reinforce color. Carat Weight Blue sapphires can range in size anywhere from a few points to hundreds of carats, and large blue sapphires are more readily available than large rubies. However, most commercial-quality blue sapphires weigh less than 5.00 carats. Fine-quality blue sapphires in larger sizes are rare, but they’re still more available than ruby. The incredible blue sapphire in the center of this ring weighs 18.79 carats. - Vagabonde Bleue Ring © 2011 Fabergé Ltd Large commercial-quality blue sapphires are more common than large fine-quality ones. As a result, size makes more of a difference in the price of fine-quality sapphire.

A fine-quality 5.00 carat blue sapphire sells for approximately five times more per carat than the same-quality 1.00 carat stone. A commercial-quality 5.00 carat stone sells for only about twice as much per carat as a 1.00 carat blue sapphire of the same quality. These examples are not meant to be exact pricing guidelines, but to illustrate how much the per-carat price can go up as the size and the quality rise. This blue star sapphire weighs 119.96 carats, making it a magnificent gem in terms of both size and quality. - Courtesy Treasured Gems and Jewels LLC/Benjamin Zucker Sapphires are available in all sizes, shapes, and colors. Learn More About Sapphire Explore sapphire history, research, quality factors, and more in the GIA Gem Encyclopedia. Cochise College P Star Rubies and Sapphires Corundum is a mineral commonly used in gemstones; known as a ruby if it is red or a sapphire if it is blue.

The type of corundum uncommonly used is in the form of a cabochon and is referred to as a ï¿½star rubyï¿½ if it is red, or a ï¿½star sapphireï¿½ if it is blue. The star in the corundum is called an Asterism, and it caused by a ï¿½silkï¿½ in the crystal called ï¿½rutileï¿½. Rutile causes a six pointed star to show when light hits the surface of the sapphire. Just as in other gemstones, the quality of the corundum is reliant on cut, color and clarity. However one must not be hasty to purchase a gem such as a star sapphire, as many of the gems in the market today are synthetic or heat treated. With a gem such as the star sapphire or the star ruby, often it is evident that the jewel has been tampered with to achieve a certain shade or color. The cut is essential to the quality of the gem because if cut too much or not enough the asterism in the sapphire will either not be bright enough or the six pointed star will almost fade altogether.

Since the cut also helps depend on the size of the gem, cutting is part of the overall quality; an incorrectly cut cabochon will dramatically decrease the size, quality, and beauty of the jewel. The worth of a star sapphire also greatly depends upon the color, as in almost all other gemstones; the deeper the color, the more expensive the corundum. A deep blue star sapphire with a clear six pointed star is very hard to come by. Corundum itself is an aluminum oxide, which means that any time aluminum ore is exposed to oxygen; it oxidizes itself and converts into the many forms of corundum. The general issue is either the star in the deep colored crystal is hazy and not well pronounced, or the well pronounced star is inside a lightly colored gemstone. In star sapphires, the majority of the gems are light blue or a grayish blue color; in star rubies the high quality gems are a dark purple red.

(Photo courtesy of Roger Weller) As you can see, the color of the ruby, which is the red variety of corundum, is a dark purplish red; most of the darker colored, higher quality star rubies on the market come from Burma, India, and Sri Lanka. Also, the star is a bit hazy and not as sharp as an extremely expensive star ruby might be, however, this particular specimen still has a great star for its color. These Sri Lanka and Burmese star rubies generally go for a high price on the gemstone market. Natural star rubies are one of the few types that are easily discernable from synthetic or heat treated gems. Heat treatment is where a gemstone is placed in a high temperature zone, much like an oven or furnace type of kiln, and is heated to change the color of the stone. These types are somewhat hard to spot in faceted gemstones sometimes, but in this type of market the rubies are more of a pink color instead of a red. They may be passed as a star ruby, but instead they are more of a pink variety of corundum sold as a sapphire that has been heat treated. (Photo courtesy of Simply Sapphires, New York).

The difference is clear between the natural and the synthetic star rubies. This has a well pronounced star, but has a bubblegum pink color to it instead of the wine color of the Burmese gem. This is the effect of the heat treatment; the color is altered from its original state in the attempt to enhance the color, and hopefully the price, of the gemstone. The color of the rest of the ï¿½starï¿½ varieties vary greatly, from pink, blue, or even black. Generally, the sapphires sold on the market consist of a light blue or grey blue variety, because of the quality of the stars, the color is sacrificed for the overall clarity of the asterism. (Photo courtesy of Simply Sapphires, New York). The light blue or grey colored star sapphires are the commonly sold star sapphires on the market. This particular specimen has a distinct asterism, is lightly colored and has streaks of darker blue enclosed in the cabochon. The higher quality gems often are a very deep color, whatever color that may be.

Often, the most expensive star sapphires are the deep blue colored specimens, regardless of the quality of the star shaped asterism in the middle. Personally, I enjoy the deep blue colored ones, but I am also fond of the black stones as well. The black stones are a very distinct, bold color to them, and look extremely attractive in both white and yellow gold. The blue variety of star sapphire is more popular than the black, since the common thoughts are that sapphires are blue. These stones are often used in jewelry, such as rings and pendants. The more vintage-style rings are very popular if they can be found. In my opinion, the vintage rings are the most attractive with this gemstone, especially if they are genuinely vintage, such as this ring. (Photo courtesy of Katie Stilwell). The pronounced star, deep blue color, enclosed in a vintage-style gold setting makes this beautiful ring a high quality specimen. The dark blue color is quite rare and when paired with the distinct but not too sharp star, if the ring were to be placed on the market it would fetch quite a hefty price.

Of course, when shopping for any type of gem, one must be also be aware of the last important aspect of the gemstone: clarity. Clarity of a star ruby or star sapphire is not as apparent as in a transparent gemstone, when the flaw is sometimes extremely clear; however they are not without their flaws. The translucent nature of the star corundum varieties are easiest to judge by their asterisms, the six pointed star that appears when it is hit by light. The asterism is caused by the mineral Rutile, which is a titanium ore that has been oxidized. Inclusions of the material Rutile into the corundum cause the asterism, and sometimes give the gem a cloudy appearance. The appearances of the cabochons depend heavily on Rutile, which is responsible for the asterism in the star sapphires and rubies, and is also the cause of the catï¿½s eye and tigerï¿½s eye in those gemstones. Rutile is commonly seen in the aluminum and silicon varieties of gemstones; such as corundum and silmanite.

These inclusions of Rutile into the corundum make the gem extremely valuable, if cut and polished correctly, and if the asterism and color are of a high quality. The asterism itself is the main focal point of the star corundum, the muted stars are not too valuable, but the sharp stars are not either. The best asterism is not too hazy, not too sharp. Though there are people who prefer them one or the other. The stones with sharper stars often sell well, as they are clearly different among the many faceted gems. (Photo courtesy of Roger Weller). Stones that are lacking a well defined star, are cloudy in appearance, have markings, striations, or other flaws in them, and are not considered a good specimen for the jewelry market. Cabochons that have striations or grooves in them will considerably drop the price, and even with a deep color to them, gems with a weak star or simply a cloudy appearance are also not considered as valuable. Some consider the striations, or the appearance of the actual crystal as quite lovely, and in that case, they can be bought quite cheap; however the majority of consumers enjoy a clear gem as opposed to a cloudy one.

Sapphire & Ruby Sapphire & Ruby “Historically”, ruby and sapphire have been at the top tier of the most important and valuable gems, their importance only surpassed by diamond. “Historically” is in quotation marks because recent history has witnessed the discovery, availability and use, by jewelry designers, of a host of varieties of beautiful colored gemstones. Collectively, these other gems are of enormous importance to the industry. In terms of value, top quality larger size rubies are much rarer and much more expensive than top quality larger size diamonds. Rubies and Sapphires Are One of the Most Prized Gems To be defined as a “gem”, a mineral (all gems are minerals with a few exceptions) must possess a combination of beauty, durability and rarity. Ruby and sapphire have almost the highest rating in all three categories. This cannot be said for any other gem. For example, a top quality, larger size emerald gets top ratings for beauty and rarity but is one of the least durable gems. A top quality, larger size diamond gets a top rating for beauty but is not as rare as a comparable size and quality ruby or emerald. Diamond is fairly durable but can be broken when struck sharply in a certain direction.

In terms of beauty, a well cut fine quality ruby or sapphire possesses both exquisite color as well as brilliance. Beauty - phenomena- star variety Durability- A combination of hardness (the ability to resist scratching) & toughness (resistant to breakage) - with 9 on the Moh’s scale, the 2nd hardest gem with very good toughness. Only diamond is harder (10 on the Moh’s scale) and diamond is not as tough. Rarity - gem quality stones are very rare What is Ruby and Sapphire? Ruby and sapphire are the identical mineral species. Both are the mineral species corundum, which is aluminum oxide in a specific trigonal crystal form. It is this combination of the aluminum oxide and the specific crystal form that imparts the durability and outstanding optical properties of corundum. Pure corundum is colorless. Corundum also exists in a very large variety of colors and the color is dependent on which type of trace (very small amount) chemical impurity is present in the gem crystal. If the impurity is chromium, the corundum crystal is a rich red and is called ruby. If the corundum crystal is a color other than red, it is called a sapphire.

If the impurity is a combination of iron and titanium, the corundum crystal is blue and is called blue sapphire. In the trade, if the word “sapphire” is used without a color prefix, it is assumed the reference is to blue sapphire. Sapphires of a color other than blue are called “fancy” sapphires. Ruby & Sapphire- A Doubly Refractive Gem Splits light beam into 2 light rays, except in 1 direction - each ray has a different velocity and different color - we see the blend of the 2 colors, in the 2 color directions Directional color change called “pleochrism” - 2 different color rays in every direction - except 1 direction has only 1 ray/1 color Burma- the major source of quality rubies. Red, clear pinkish Red, purplish Red - Mong Hsu- the largest source. Has a bluish core and whitish clouds that must be “burned” out into a beautiful stone. Glassy feather inclusions. - Mogok- Known for its most prized and rarest pigeon-blood (pure red) color variety. Short rutile silk and well formed crystal inclusions.

Vietnam- fine purplish Red, purplish pinkish Red. Thailand- dark Red, brownish Red, Saturn inclusions. Tanzania- highly saturated Red with medium tone (not too light, not too dark). Madagascar- large source of very good color but highly fractured rubies. This is the major source for glass filled rubies. Afghanistan, Cambodia, Kenya, and Sri Lanka- Some quality stones but not major sources. India- low grade stones. Six ray stars are the most common. Twelve ray stars are rare. The star phenomenon (“asterism”) is caused by intersecting parallel bundles of fine rutile needles. Each bundle of needles reflects a straight, thin band of light in a direction perpendicular to the long direction of the needle bundle. If a stone has these bundles of needles it must be cut in cabochon style to focus and show the star. A combination of a good star & a good body color is rare and highly prized. Star Sapphires- The best are from Sri Lanka. Other sources are Burma, Thailand and Cambodia. Star Rubies- The best are form Burma. Other sources are Sri Lanka, Thailand, Cambodia and India.

Synthetic corundum can be identified only by magnification. Synthetic corundum has the same chemical and physical properties as natural corundum. Doublet- A synthetic sapphire or ruby bottom with a thin slice of the natural stone glued on top. When viewed from the top (even with magnification), the stone will appear to be natural. The purpose is deception. Imitation- Any stone that has the same appearance as the stone it imitates but has different properties. Examples: Red garnet and red tourmaline for ruby. Benitoite, kyanite and tanzanite for blue sapphire. Grossular yellow garnet and citrine quartz for yellow sapphire. The purpose is deception. If a rough stone has good transparency and clarity (not highly included), it is normally cut in a faceted style to impart brilliance to the cut stone. If a rough stone is not transparent and/or has poor clarity (a lot of inclusions) but an attractive color and more or less uniform color, it is normally cut in cabochon cutting style. Some cabochons can be very beautiful.

The great majority of good quality, center stone size, facet grade sapphires and rubies are cut in the mixed cut style, which is a brilliant cut on the top (crown) and a step cut on the bottom (pavilion) of the stone. The mixed cut style provides a compromise by providing brilliance while saving carat weight. When you shop for a faceted sapphire or ruby, you will have the largest selection to choose from and obtain the best value and combination of size and beauty for your money, if you shop for stones in the mixed cut style. If you are shopping for a center stone size cabochon sapphire or ruby, you should be aware that these stones were very likely not good enough quality to cut in a faceted cutting style. Therefore this fact should be reflected in the pricing of the cabochon. Star sapphires and star rubies are an exception to this guideline. Heat Treatment- The overwhelming majority of rubies and sapphires are heat treated to improve color and clarity. The heat improves clarity/transparency and color by dissolving whitish clouds of rutile silk.

In the case of Mong Shu Burmese rubies, borax (sodium borate) flux is used when heating to heal fractures in the stone that reach the stones surface. This improves both the durability and clarity of Mong Shu rubies. The flux helps melt the ruby material in the walls of the fracture which then seals shut upon cooling. Any glassy flux residue on the surface is washed away with acid. The heating also eliminates the whitish rutile clouds and the bluish inner core of Mong Shu rubies and the result is a beautiful red ruby. The overwhelming majority of fine ruby stones of 3 carats and under, in the marketplace are of Mong Shu origin. It is accepted and assumed in the trade, that unless specifically specified as unheated, that all rubies and sapphires are heat treated. Thus, there is no routine disclosure on lab certificates, stone invoices, stone boxes and envelopes. As such, heat treatment does not “lower the value” of a stone. There is a niche market that will pay a higher price for unheated stones. Thus, it can be said that a fine unheated specimen, of center stone size, will fetch a higher price.

This should be verified by lab testing and a certificate. Beryllium Treatment- This treatment is now in wide use to improve the pale color of yellow sapphires and to turn some yellow sapphires into orange sapphires. Beryllium treatment can also be used to improve the color of some blue sapphires and some rubies. Beryllium treated stones are less valued than non-beryllium treated stones and are (and should be) priced accordingly. It is the accepted standard to label beryllium treated rubies, blue sapphires and orange sapphires, as “beryllium treated.” As the majority of yellow sapphires (and orange sapphires), in the current market, are beryllium treated, it is assumed they are beryllium treated unless otherwise specified. Diffusion Treatment- The primary use, of this treatment, is to turn a colorless sapphire or pale blue sapphire into a good or top color blue stone. A mix of aluminum oxide powder and iron and titanium trace transition elements is melted and diffused into a thin surface layer of the faceted stone. Although not common, there are diffusion treated rubies.

Routine disclosure is a must for this treatment. Dyeing- Obviously this should be disclosed, but rarely is because the purpose is almost always deception. Found more in rubies than other corundum varieties. Glass Filling- Glass is melted to fill the fracture cavities of highly fractured stones. This results in a tremendous improvement in clarity and turns commercially non-viable stones into saleable stones. This is most commonly employed with Madagascar rubies, which are highly fractured. Disclosure is a must. Glass filled rubies (and sapphires) have a much lower value than un-filled stones and should be priced accordingly. Gemstone Definitions and History Mineralogy is the science or study of minerals including crystalline structure and chemical composition, mineral origin, occurrence, and associations. A mineral is naturally occurring homogeneous solid with a definite, but not fixed, chemical composition, characteristic crystalline structure, usually of an inorganic origin. Naturally occurring rules out products made in laboratory. Even though synthetic gem material has identical chemical, physical, and optical properties to the natural, they are not termed minerals or gems, but rather synthetics or created stones. Homogeneous is of one thing or cannot be physically subdivided into anything simpler.

Solid implies material that is neither liquid or gaseous, for example ice is a mineral but water and steam are not. Chemical composition and crystalline structure refer to the atoms or ions that are arranged in a regular array in three-dimensions. Quartz has a fixed chemical composition, SiO2, arranged in a hexagonal pattern and referred to as a pure substance. The mineral group, olivine, has a definite but not fixed composition of (Mg, Fe)2SiO4. A magnesium silicate, Mg2SiO4, is at one extreme, while the iron silicate, Fe2SiO4, is at the other extreme. The iron and magnesium are close enough in size to substitute for one another in the crystal lattice. The more magnesium-rich olivine is known as forsterite and if it is gem quality, termed peridot. A characteristic crystalline structure means there is an internal structural framework in which atoms are arranged in a geometric pattern. Inorganic origin has been a necessary component in past definitions of minerals. However, according to Nickel (1995) a mineral is an element or chemical compound ...formed as a result of geological processes (p. 689).

Nickel goes on to clarify ...biogenic substances are chemical compounds produced entirely by biological processes without a geological component (e.g., urinary calculi, oxalate crystals in plant tissues, shells of marine molluscs, etc.) and are not regarded as minerals. However, if geological processes were involved in the genesis of the compound, then the product can be accepted as a mineral. Thus, modern shells of marine molluscs are not mineral but calcite and aragonite constitutents that result from compaction of these marine organisms in limestones are considered mineral. Mineraloid "Crystalline" restricts the material defined as mineral and mostly excludes liquid and organic material. As such, naturally occurring material that is lacking a portion of the mineral definition criteria can be called mineraloid. Opal is a mineraloid because the crystalline structure is not well defined, whereas modern marine shell material and kidney stones are mineraloid because they are biogenic substances or organic in origin. Some gem material, such as pearl and mother-of-pearl, hold a kind of dual citizenship, called mineral by some and mineraloid by others.

Although most pearls are created by bivalve mollusks, some are formed in caves as calcium carbonate precipitates around bat bones or some other small grains on the floor of the cave. Classification scheme for minerals and gem materials The system used by gemologists to classify gems includes: group, species, variety, organics, and synthetics (Hurlbut & Kammerling, 1991, pg. 5). The group is two or more chemically related gem species that have similar properties and structures (e.g., feldspars or garnets). The inorganic species are defined by chemical composition and crystalline structure (e.g., albite feldspar or tsavorite garnet, corundum or beryl). Organic gem material species are divided on composition and can include pearl, coral, shell, amber, jet (coal), and ivory. Species are further subdivided into varieties (e.g., moonstone albite feldspar, ruby and sapphire corundum, or emerald and aquamarine beryl). These classifications can be based on color, color distribution, transparency, optical phenomena, or any combination.

Examples of subdivisions based on color are: the red variety of corundum is termed ruby, the blue variety is sapphire, and the many other colors of gem quality corundum are termed fancy sapphires, with varieties designated by color (e.g., green sapphire, pink sapphire). The mineral species beryl is found in a deep green variety, emerald; a medium blue variety, aquamarine; a pink variety, morganite; a colorless or "white" variety, goshenite; a yellow variety called golden beryl or heliodor; and a red variety, red beryl. Examples of subdivisions based on color distribution are onyx and agate. Onyx is a black and white, fine-grained variety of chalcedony, a microcrystalline quartz. Agate has many color possibilities with curved or angular banding. Transparency, or the ability of the material to transmit light, can be highly transparent in colorless quartz, medium transparency or translucent as in the reddish chalcedony called carnelian, or a low transparency, that is nearly opaque, in the red variety of quartz called jasper. Gem material can also be subdivided based on optical phenomenon.

One example is asterism or the star effect in star ruby, shown by intersecting bands of light on a curved surface. Gems In the U.S., the term "gem" can only be used for natural material, not synthetics. The high value of gems and a demand for perfection lead to synthesizing materials by the end of the nineteenth century. Corundum and spinel were the first gem materials to be created large enough for faceting. Rutile, quartz, alexandrite, and opal are more synthesized gem materials to follow. Industrial grade diamonds were first synthesized in 1953 and gem quality by the 1970s. When the lab created material is identical to the optical, physical, and chemical properties of its natural counterpart it is called synthetic. Lab created materials that mimic natural gems, that is resemble but not contain identical properties, are termed imitations or simulants. Common imitations are lab created glass and plastics, produced to resemble a great number of different gemstones. Another example of an imitation is synthetic cubic zirconia or CZ, originally marketed as a diamond simulant. CZ resembles diamond but does not have the same properties as diamond.

A lab created material could be both a synthetic and imitation depending on how the material is marketed, for example purple corundum could be created and termed synthetic purple sapphire or used to resemble purple quartz and termed an amethyst imitation or simulant. Visit the USGS webpage on Synthetic or Simulant from the United States Geological Survey (USGS), . Highly Recommended Readings!

Question: What is a very hard, naturally-occurring mineral, of which ruby and sapphire are gem quality varieties?
Answer: Corundum